dart - 在Dart中将可观察属性从一个对象转发到另一个

Question

我想检测一个对象的属性更改，然后将该值转发(或重新计算该值并将结果传递)给另一个对象的属性。我从文档中看到了演示值(value)转发的示例:

class MyModel extends Observable {
  StreamSubscription _sub;
  MyOtherModel _otherModel;

  MyModel() {
    ...
    _sub = onPropertyChange(_otherModel, #value,
        () => notifyPropertyChange(#prop, oldValue, newValue);
  }

  String get prop => _otherModel.value;
  set prop(String value) { _otherModel.value = value; }
}

但是我不知道从哪里获取oldValue和newValue。
我认为这些应该作为参数传递给onPropertyChange的回调(第三个参数)，但事实并非如此。回调不提供任何参数。这是疏忽还是我错过了什么？

Answer 1

import 'dart:async';
import 'package:observe/observe.dart';
class MyOtherModel extends Object with Observable {
  @observable
  String value;
}

class MyModel extends Object with Observable {
  StreamSubscription _sub;
  MyOtherModel _otherModel = new MyOtherModel();

  MyModel() {

    ///...
    _otherModel.changes.listen((crs) {
        crs.forEach((PropertyChangeRecord cr) => 
             notifyPropertyChange(#prop, cr.oldValue, cr.newValue));
    });
  }

  String get prop => _otherModel.value;
  set prop(String value) => _otherModel.value = value;
}

void main() {
  MyModel m = new MyModel();
  m.prop = 'bla';
  m.changes.listen(print);
  // initiate change notification
  Observable.dirtyCheck();
}

输出

[#<PropertyChangeRecord Symbol("value") from: null to: bla>]